Almost half of Thailand's eligible voters failed to cast their ballots in the country's Senate elections - the second major vote in recent months.

Sunday's poll took place under a new constitution drafted by the military, which ruled for 18 months after ousting PM Thaksin Shinawatra in a coup.

The constitution gives wider powers to the upper house - but nearly half of its members are appointed, not elected.

This may be why turnout was reportedly lower than expected, say observers.

"We are satisfied with the voter turnout although the number was lower than our target [of 70%]," said Praphan Naiyakovit, who sits on the election commission, according to the Thai News Agency.

Votes are now being tallied after the election, in which about 56% of the electorate participated.

The election may have been overshadowed by the frenzy of excitement that followed the return from exile of Mr Thaksin on Thursday, says the BBC's Jonathan Head in Bangkok.

Election fatigue?

Another reason for voter apathy could be that the real electoral drama happened over two months ago, when the party led by Mr Thaksin's allies stunned the military government by winning the contest for the lower house, our correspondent says.

In addition, this election is only for just over half the Senate - as 74 of the 150 senators had already been appointed by a committee of judges and top bureaucrats.

This provision was inserted into the new constitution by the military-appointed drafting committee last year as a kind of guarantee that there would be no repeat of the extraordinary concentration of power in the hands of an elected prime minister, as happened when Mr Thaksin was in office.

The Senate has considerable muscle - it can sack cabinet ministers and the heads of all the key political oversight bodies, and it can impeach the prime minister.

Its members are supposed to have no ties to political parties.

But under the Thaksin administration, the upper house was filled with people either allied to him or related to his ministers, and correspondents said it was ineffective in challenging his governments.

The 74 newly-appointed senators appear to be far closer to the outgoing military government than the one now headed by Mr Thaksin's party.

This could for the first time produce some genuine battles between the lower and upper houses, our correspondent says.